add statement cache in connection.

hide methods from api

spec/std/db/dummy_driver.cr

@@ -21,7 +21,7 @@ class DummyDriver < DB::Driver
       @@connections.try &.clear
     end
 
-    def prepare(query)
+    def build_statement(query)
       DummyStatement.new(self, query)
     end
 
@@ -30,6 +30,7 @@ class DummyDriver < DB::Driver
     end
 
     protected def do_close
+      super
     end
   end
 

spec/std/db/statement_spec.cr

@@ -80,4 +80,15 @@ describe DB::Statement do
       stmt.closed?.should be_false
     end
   end
+
+  it "connection should cache statements by query" do
+    with_dummy do |db|
+      rs = db.query "1, ?", 2
+      stmt = rs.statement
+      rs.close
+
+      rs = db.query "1, ?", 4
+      rs.statement.should be(stmt)
+    end
+  end
 end